1. Field of the Invention
The present invention relates to a method of manufacturing a lead frame, and more particularly to a method of manufacturing a lead frame to produce leads in a fine pattern at a reduced cost while preventing the leads from being deformed in the fabrication process and allowing the leads to be plated with ease.
2. Description of the Related Art
As the degree of integration of semiconductor devices including ICs, LSI circuits, etc. increases and those semiconductor devices have a greater number of terminals, more and more lead frames manufactured by TAB (Tape-Automated Bonding) are replacing conventional lead frames of a metal sheet of copper or the like which is pressed or etched to form a plurality of integral leads, as providing electric leads of electrodes of the semiconductor devices.
TAB lead frames comprise a tape of a heat-resistant synthetic resin such as polyimide and a plurality of leads of copper or the like supported on the tape, as disclosed in Japanese laid-open patent publication No. 5-198618.
Heretofore, a TAB lead frame is manufactured as follows: A metal film of copper or the like is deposited on a flexible tape base of an insulating synthetic resin such as polyimide. Then, the metal film is selectively etched into a predetermined pattern of leads. The conventional manufacturing process has imposed a limitation on efforts to produce a fine pattern of leads because the metal film has a poor affinity for the tape base of an insulating synthetic resin. Therefore, it has been difficult to form a film, such as a copper film, of thin dense leads.
Furthermore, the width of leads cannot be reduced beyond a certain limit, and the leads are subject to a phenomenon known as side etching. Since the tape base is not right, but flexible, is managed and assembled tool to tool, and is difficult to handle as it tends to be easily curled, so that it is difficult to form leads on the tape base. These shortcomings have also been responsible for thwarting attempts to make finer lead patterns.
Since leads are formed on an insulating tape base and are insulated from each other thereby, if inner or outer leads are to be electroplated with metal for bonding, then it is necessary to provide a circuit for applying a potential to the leads only for electroplating the leads, and then remove the circuit after the leads are electroplated. Such a process is disadvantageous in that an extra space for providing the circuit therein is required, the process needs a large number of steps, and the resultant lead frame is highly costly.
The synthetic resin of the tape base or carrier for the leads is expensive, and is wasteful because it is mostly removed eventually. This has prevented the cost of the conventional lead frames from being lowered.
Tapes for use in TAB are manufactured by a tool-to-tool tape carrier system. Therefore, they can be mass-produced in few types. However, the TAB tapes do not lend themselves to a production scheme for producing many types in small quantities, which has recently been considered to be more important than before in the art. The sizes, particularly widths, of TAB tapes and circuit patterns thereon are limited by production equipment standards. Accordingly, the TAB tapes find it difficult to meet a variety of needs which will be growing in the art.